Fix person link widget after validation error
Fix person link widget after validation error

When adding a new person it was serialized as a PersonLink if the form
needed to be re-rendered, but after submitting the form again this
failed because we now tried to parse it as an existing EventPerson
instead of a new person

mvidalgarcia/indico,mvidalgarcia/indico,mic4ael/indico,ThiefMaster/indico,ThiefMaster/indico,OmeGak/indico,indico/indico,indico/indico,mvidalgarcia/indico,OmeGak/indico,indico/indico,OmeGak/indico,pferreir/indico,mic4ael/indico,OmeGak/indico,DirkHoffmann/indico,DirkHoffmann/indico,pferreir/indico,pferreir/indico,indico/indico,mic4ael/indico,ThiefMaster/indico,DirkHoffmann/indico,mvidalgarcia/indico,DirkHoffmann/indico,pferreir/indico,ThiefMaster/indico,mic4ael/indico